I have time after Cartagena and Medillen to fit in one more city, what would you choose?
Depends on what you are looking for and where you have to take your flight back.
Cali is to learn and dance Salsa - If you don't like Colombian music don't go there. It's great to have Colombian Party and beautiful girl. Otherwise, the city is not really interesting
Bogota is a capital. You have more monuments (Candelaria, Monserrate), good restaurant, day trip and crazy techno party. But the weather is not great.
Hope it helps!
Bogota would be my choice. Cali is nice, however I would never choose it over Bogota which has much more to offer. I like Cali for its party's, however you can easily get that in Bogota.